1. Field of the Invention
This invention relates to a photovoltaic element with a novel structure capable of generating a photovoltaic effect with good efficiency by stimulation from electromagnetic waves such as UV-rays, visible light, and IR-rays, particularly sunlight.
2. Description of the Prior Art
In the prior art, studies have been made about those photovoltaic elements employing single crystal or polycrystalline silicon (written as C-Si), GaAs, InP, CdTe, and CuInSe.sub.2, and some of them have been used in practical applications. Particularly, in recent years, hydrogenated amorphous silicon (written as "a-Si:H") has attracted attention as a material for forming a photovoltaic element, with the advantage of being capable of p and n type doping, in addition to easy area enlargement due to the use of amorphous material. The reason why photovoltaic elements employing a-Si:H have actively been studied is that a-Si:H inherently provides various advantages as mentioned below:
(1) Because a-Si:H is amorphous in nature, the selection rule is not valid for its light absorption; therefore its light absorption coefficient is very great. It exhibits sufficient light absorption even with a thickness of about 1/200 of that of conventional C-Si.
(2) Since sufficient light absorption can be exhibited even with a thin thickness, only small amounts of starting material for preparation of thin film are required, which will lead to conservation of energy.
(3) p and n doping control can be accomplished by a relatively simple method. For example, if a gas such as PH.sub.3 or B.sub.2 H.sub.6 is mixed into the starting SiH.sub.4 gas, P or B can be sufficiently incorporated into the thin film during the glow discharge decomposition reaction, whereby a n-type or p-type semiconductor thin film can be formed.
(4) Due to lower production cost and production energy, cost repayment and energy repayment of the photovoltaic element itself can be done within a short period of time, whereafter a net energy gain will be obtained.
As for the structures of the known photovoltaic elements employing a-Si:H, there are (1) the Schottky barrier type, (2) the metal/insulating layer/aSi:H type, (3) the p-i-n junction type of a-Si:H, etc. Concerning the former two types, due to utilization of a metal as a surface electrode, light reflectance from the surface is generally large and the quantity of light reaching the semiconductor layer is limited by the species of metal employed and the deposited thickness of the metal, thereby lowering of efficiency. Accordingly, the structure which is now being developed with good progress is the p-i-n type photovoltaic element of type (3). The element structure, including its electrodes, has been reported variously and is typically of the metal/n-i-p structure/ITO/glass type or the ITO/n-i-p structure/metal type. One of the most important tasks for photovoltaic element improvement is that of photoelectric transducing efficiency. According to a recent report, there has been obtained a thin film solar battery with an efficiency of over 10 %. Such improvement in efficiency may be attributable to removal of impurities from the i layer and improvement in spectral sensitivity. By imparting a "window effect" by use of an a-SiC or a-SiN layer on the light irradiating side of an element, improvement in spectral sensitivity as well as prevention of inverse diffusion of carriers may be effected. Also, as a means for preventing contamination by an impurity in the i layer, it has been proposed to utilize a three-compartment separation type reaction device. While utilization of a-SiC or a-SiN for improvement of spectral sensitivity is a great step forward, even use of these as a window layer will make it possible only with difficulty to broaden the spectral sensitivity to include the entire wavelength region of sunlight or a room lamp. In other words, a wide band gap material with no light absorption is ideally desirable as the window layer, but use of a completely transparent film of a-SiC or a-SiN as the window layer has so far resulted in lowering of photoelectric transducing efficiency of the element as a whole. Thus, a problem remains in the presently available a-Si:H photovoltaic element with respect to blue sensitivity. Further, for improvement of transducing efficiency of photovoltaic elements, it is also necessary to lower the series resistance of the element. For these reasons, even in the case of a-SiC or a-SiN, in spite of much efforts paid to p and n doping control, control of optical band gap (Eog), and lowering of resistance, problems remain to be solved.